1. Field of the Invention
The present invention relates to a radiation imaging apparatus, a radiation imaging system, a method for radiation imaging, and a storage medium.
2. Description of the Related Art
There has been commercially available a radiation imaging system using a radiation generator which irradiates an object with radiation, a radiation imaging apparatus which generates a clear radiation image by performing image processing for the radiation image obtained by digitizing a radiation image as a radiation intensity distribution, and an image processing apparatus. In such a radiation imaging system, the radiation irradiation apparatus irradiates an object with radiation, and the radiation imaging apparatus transfers obtained radiation image data to an image processing apparatus such as a control computer for image processing and saving. The image processing apparatus causes a display apparatus such as a display to display the processed image.
The radiation imaging apparatus uses a sensor array having a two-dimensional array of pixels each including a conversion element which converts radiation into an image signal charge (electrical signals) and a switch element such as a TFT which transfers electrical signals to the outside. Performing matrix driving using switch elements such as TFTs transfers the signal charges converted by the conversion elements to a readout image processing apparatus, thereby forming an image from a readout charge amount.
Each conversion element on the sensor array directly or indirectly generates signals upon being irradiated with radiation. In a sensor designed to indirectly generate signals, the conversion element of each pixel detects the visible light converted from radiation by the phosphor instead of directly detecting the radiation. In either the direct or indirect sensor, each pixel generates and accumulates a certain level of a signal even without any radiation irradiation. This signal is called a dark charge here.
Dark charges have different characteristics in the respective pixels on the array. Superimposing dark charges on image signal charges (electrical signals) originating from radiation irradiation will degrade the image quality in such a manner that offsets are added to the image. To prevent this, it is a general practice to discharge (reset) accumulated dark charges by turning on the switch elements of the respective pixels periodically and/or immediately before radiation irradiation.
When resetting dark charges, if image signals are superimposed on the dark charges, it is not possible to separate them from each other and extract only the dark charges. Resetting dark charges during radiation irradiation or in the interval from the end of radiation irradiation to image signal readout operation will result in losing image signals. It is therefore necessary to exclusively execute resetting of dark charges and radiation irradiation. For this purpose, there is provided a mechanism for synchronizing the radiation imaging apparatus with the radiation irradiation apparatus. Japanese Patent Laid-Open No. 2003-33340 discloses a radiation imaging system including such a mechanism.
Starting to irradiate the pixels on the sensor array with radiation will generate charges inside the pixels and flow out to the bias lines connected to the respective pixels, resulting in a rapid increase in the amount of current in each bias line. For example, Japanese Patent Laid-Open No. 2009-219538 has proposed a radiation imaging apparatus which detects the start or the like of radiation by detecting a change in this amount of current.
In addition, as described above, since dark charges are always generated on the sensor array, it is necessary to periodically reset dark charges. For this reason, as shown in FIG. 1, the sensor array is configured to detect a change in the amount of current in each bias line while performing the reset scanning (TC101) of sequentially driving the respective rows (L0 to L10, . . . ) on the sensor array to turn on the switch elements so as to reset charges in the respective pixels connected to the corresponding rows. At the moment of the detection of the start of radiation, the sensor array stops the reset scanning on the row on which it has executed reset scanning (TC102), and turns off the switch element to shift the sensor driving state to the state of accumulating operation for image signal charges originating from radiation (TC103). In this state, the sensor array detects radiation.
Upon completion of radiation irradiation, the sensor array sequentially drives the respective rows to turn on the switch elements to shift the sensor driving state to the state of output operation for image signal charges, thereby reading out the radiation image signals accumulated in the respective pixels (TC104).
In this case, each pixel on the row in the process of reset operation at the time point (TC102) when the start of radiation irradiation is detected lets part of effective image signal charges generated by radiation irradiation flow out because the switch element is in the ON state.
In addition, in the radiation irradiation apparatus, if the dose of radiation does not instantly rise to the operating level at the start of radiation irradiation but gradually rises, the radiation imaging apparatus may detect the start of irradiation with a delay. In this case, the sensor array performs reset scanning on a plurality of rows (L2 to L7 in the case shown in FIG. 1) from the row corresponding to the time point when the radiation irradiation apparatus has actually started irradiation to the row corresponding to the time point when the radiation imaging apparatus has detected the start of irradiation. As a consequence, effective image signal charges accumulated over a plurality of rows by radiation irradiation partly flow out.
As shown in FIG. 2, the pixel values on the rows (L2 to L7) from which effective charges flow out are smaller in charge amount than those on preceding and succeeding rows (L0, L1, L8 to L10, . . . ), and hence lack in reliability. This makes it necessary to perform correction processing such as data interpolation. For example, the apparatus disclosed in Japanese Patent Laid-Open No. 2011-249891 is configured to wait for the detection of the start of radiation irradiation while sequentially performing the above reset scanning (TC301) on rows which are not physical adjacent to each other as shown in FIG. 3. According to this, there is provided a method of preventing rows (L2, L4, L6, and L8) on which data defects occur at the time of the detection of radiation from consecutively appearing, as shown in FIG. 4, and interpolating data on the defective rows from data on the preceding and succeeding normal rows, thereby improving the correction processing accuracy of image data. If, for example, a defective row is L2, interpolation processing is performed by using the preceding and succeeding normal rows (L1 and L3).
In addition, the radiation imaging system is required to be capable of displaying a captured image immediately after the execution of imaging in order to quickly determine whether imaging has been normally performed (re-imaging is required). However, a captured image requires various types of image correction processing such as offset correction processing for correcting offset components in the image and the transfer time required to transfer the image to the display device, resulting in a delay time (display delay time) from imaging to the display of the image.
For example, Japanese Patent Laid-Open No. 2012-152340 discloses a method of transferring a preview image to a display device upon offset correction to reduce the display delay time.
As disclosed in Japanese Patent Laid-Open Nos. 2009-219538 and 2011-249891, when the radiation imaging apparatus is configured to detect the start of radiation irradiation by itself, image data deteriorates due to data defects at the time of the detection of radiation on reset rows and their nearby rows at the time of the detection of the start of radiation irradiation. When displaying an obtained image as a preview image in advance upon reduction or the like, displaying an image deterioration at the time of the detection of radiation may make it impossible for the operator to determine whether imaging has been normally performed. This may make it necessary to perform re-imaging.
For this reason, in order to correct a deterioration in image data, it is necessary to perform image correction processing. If, however, image correction is also performed for a preview image, a time loss for correction processing occurs. This makes it impossible to quickly display an image.